Hitherto, aluminum electrolytic capacitors of this kind have ordinarily been constituted by winding an insulating paper and an aluminum foil processed by electrolytic etching in an electrolytic solution containing chloric ion to enlarge its effective surface area.
The surface areal enlarging action of an electrode foil by the electrolytic etching is influenced by a number of factors, for example, etching current waveform, current density, kind of etching solution, concentration, temperature, and kind and concentration of additives, and furthermore, pre-etching process, inter-stage process, post-process, kind of aluminum raw foil, and so on.
Among them, the investigation concerning the current waveform has been carried out since long time before, and ordinarily, DC current, AC current, and pulse current are used in industry.
The etching method utilizing pulse current waveforms has also been investigated in various ways, and as the methods using pulse current waveforms in an electrolytic solution containing chloric ions, British Patent No. 1,169,234, Japanese unexamined published patent application No. Sho 57-132322, and others are known.
Also, as the method using pulse current waveforms in an electrolytic solution containing chloric ions and chromic acid ions as an additive, Japanese examined published patent application No. Sho 55-36280 is known.
In the method of adding chromic acid ions stated in the Japanese examined published patent application No. Sho 55-36280, there were drawbacks that the etching waste lye was not only harmful for human bodies but also its concentration in the waste lye was regulated by law to be 0.05 ppm or below and hence the pollution elimination process of this waste lye using such as the reduction-hydroxide-precipitation method or the ion-exchange method consumed labor and time, and brought danger, resulting in cost-increase.
In this method, due to the use of chromic acid ion having a high oxide film production action, those parts except initially produced corrosion holes are protected by the film, further production of those corrosion holes is difficult to occur and only those initially produced corrosion holes proceed deeper into the inside of the aluminum foil. As a result of this, the depth of the corrosion holes becomes 100.mu.-150.mu., and for those aluminum foils of the thickness less than 150.mu. the strength against folding of the aluminum foil becomes weak, and hence it was not suited to its industrialization.
And, the used current densities being less than 0.5 A/cm.sup.2, if it is attempted to increase the current density to improve the productivity, the etching formation becomes non-uniform, and the surface enlarging factor decreases, and then the etching time cannot be reduced, hence there was a drawback that there is a limitation to the improvement of the productivity.